Cheap And Cheerful Transistor Tester(CD4011B)

2016-01-21 19:24  
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This article describes a Cheap And Cheerful Transistor Tester (CD4011B). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: CD4011B.

By using a simple visual indicating system, this small transistor tester allows you to run a quick ‘go/non-go’ check onNPNas well asPNPtransistors. If the device under test is a workingNPNthen the greenLED(D1) will ?ash, while the red counterpart will ?ash for a functionalPNPdevice. However if the transistor is shorted, bothLEDs will ?ash, and an open-circuit device will cause theLEDs to remain off. The circuit is based on just one CD4011B quadNANDgate IC, four passive parts and twoLEDs. The fourth gate in the IC is not used and its inputs should be grounded.

Alternatively, you may want to connect its inputs and output in parallel with IC1.C to increase its drive power to the transistor test circuit. IC1.A and IC1.B together with R2, R3 and C1 form an oscillator circuit that generates a low-frequency square wave at pin 4. This signal is applied to the emitter of the transistor under test as well as to inverter IC1.C. The inverted signal from IC1.C and the oscillator output then drive the test circuit (LEDs, device under test, R1) in such a away that the voltage across that part of the circuit is effectively reversed all the time.

Figure:1 Cheap And Cheerful Transistor Tester Circuit Diagram

Figure 1 Cheap And Cheerful Transistor Tester Circuit Diagram

For example, with anNPNtransistor under test, when pin 10 is High and pin 4, Low, current ?ows throughLEDD1 and the forward biased transistor. However, no current will ?ow when pins 10 and 4 change states, since the transistor is then reverse-biased. The greenLED, D1, will therefore ?ash at the rate determined by the oscillator. As you would expect to happen, aPNPtransistor will be forward biased when pin 10 is Low and 4, High, enabling current to ?ow through the redLEDin that case.

A supply rail of around 3 V (two series connected 1.5-V batteries) should be adequate. To prevent damage to the transistor under test, supply voltages higher than 4.5 V should not be used. Because theLEDcurrents are effectively limited to a few mA by the output of IC1.C (also slightly dependent on the supply voltage), it is recommended to use high-efficiency devices for D1 and D2.



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